During the manufacture of fiberboard in accordance with the dry method, the raw material is generally disintegrated by defibering, and the defibered material is then normally dried with drying gas in one or more steps, and glued. The dried material is then formed into a fiber mat, which is then hot pressed to provide a finished board. Multi-layer fiberboard is manufactured in a corresponding manner.
The addition of glue is required in these processes in order to provide a fiberboard having sufficient strength. The glues utilized for this purpose are normally thermosetting glues, such as urea resins or phenolic resins.
During these known fiberboard manufacturing methods, the glue is mixed with the fiber material after its defibration, and the material is then dried with hot air. The hot air so utilized generally has a high ingoing temperature, for example, of about 160.degree. C., so that during the drying process the glue can become undesirably overheated, and thus harden, at various locations, causing the hardened glue to lose its adhesive capacity. In order to then compensate for this loss of effectively active glue, excessive amounts of glue must therefore be added, which, in turn, results in a significant cost increase due to the required unnecessarily high glue consumption. The cost of the glue thus constitutes one-quarter to one-third of the total cost of the finished fiberboard, thus resulting in a considerable increase in the overall manufacturing cost, and a corresponding deterioration of the manufacturing economy.
In another method of fiberboard manufacture, the fiber material is first dried in a drier with a high temperature drying gas, for example at about 200.degree. C. The dried fibers leaving the drier are then mixed with glue in a mixer. During this admixture, the glue is mixed with an approximately equal amount of water. The addition of glue is in the magnitude of about 10% of the dry weight of the ready mixture. In order to compensate for the water which is added with the glue, the fibers must therefore be correspondingly overdried. This overdrying, however, causes the fibers to become too dry, and thus have a poor adhesive capacity to the added glue. Instead of being uniformly distributed over the fibers, portions of the glue will therefore form small lumps distributed within the fiber material, or so-called glue spots. This non-uniform distribution of the glue must therefore be compensated for by an increase in the glue addition, which results in a deterioration of the quality of the finished fiberboard.
Another prior art variation of the dry method of fiberboard production is set forth in Swedish Patent No. 462,707. This patent discloses a method in which fiberboard is manufactured using two drying steps with intermediate mixture with a binding agent, and in which the second drying step is conducted at a lower temperature than the first drying step.
U.S. Pat. No. 5,034,175 discloses yet another process for manufacturing fiberboard which, in this case, discloses a two-stage drying method which includes transferring the dried lignocellulose-containing material from the drying steps directly to a forming station where the fiber web is formed and in which the temperature of the dried lignocellulose-containing material is maintained during that transfer.